viridis: Default Color Maps from 'matplotlib'

Port of the new 'matplotlib' color maps ('viridis' - the default
-, 'magma', 'plasma' and 'inferno') to 'R'. 'matplotlib' <http://matplotlib.org/> is a popular plotting library for 'python'. These color maps are designed
in such a way that they will analytically be perfectly perceptually-uniform,
both in regular form and also when converted to black-and-white. They are
also designed to be perceived by readers with the most common form of color
blindness.